Conventionally, coding schemes typified by H.264 standardized by MPEG (Moving Picture Experts Group), ITU-T (International Telecommunication Union, Telecommunication Standardization Sector) and the like have been widely used in the area of moving image encoding. To reduce the amount of data, the prediction coding is adopted for these coding schemes. In encoding, differences between an image predicted by a certain method (prediction image) and its original image are encoded and in contrast, in decoding, a decoded image is obtained by adding the differences to a prediction image.
A frame of image is formed by e.g. brightness and two color differences. This format is referred to as YCbCr. A moving image is composed of a sequence of frames of images. In the present digital terrestrial broadcasting, about 30 frames per second are transmitted. Based on this, the techniques for producing a prediction image are roughly classified into two types. The first is a technique such that one frame of image is independently used to perform the prediction within one picture (intra-frame prediction or intra-picture prediction). The other is a technique such that the feature of a sequence of images is used to perform the prediction between pictures (inter-frame prediction or inter-picture prediction). In general, there is a high correlation between frames in a moving image. Therefore, the inter-picture prediction is higher than the intra-picture prediction in the encoding efficiency. However, an image encoded only by the intra-picture prediction is necessary for the top of a moving image, the improvement in the random accessibility in replay and the recovery at occurrence of an error, and in general, such image is inserted once every 0.5 to 2 seconds approximately.
The intra-picture prediction technique and the inter-picture prediction technique are described in Non Patent Documents 1 and 2 respectively. In the Non Patent Documents, the coding process is performed on an image in processing units referred to as macroblocks; the image is divided into the macroblocks of 16 pixels×16 lines. The unit of producing a prediction image may be e.g. 8×8 and 4×4 into which the inside of each macroblock is further finely divided (the unit of producing a prediction image is hereinafter referred to as “block” in this specification).
In the inter-picture prediction, an image already encoded/decoded is used to produce a prediction image. During it, reference index and motion vector showing, for each block, which image to perform the prediction on, and which position in the image to perform the prediction from are encoded. The reference index shows an image to use for the prediction, which is assigned as an image one frame before the current image to Index 0, and an image two frame before the current image to Index 1, for example. The motion vector is represented by a set of values of difference between the position of the current block and the position of an image used for prediction, and it consists of two values of a horizontal component and a vertical component because an image is two-dimensional information. In the Non Patent Document, only a vector of brightness is encoded in a bitstream.
In contrast, in the intra-picture prediction, a prediction image is produced from pixels adjacent to the block following a certain rule. In the Non Patent Documents 1 and 2, nine methods including a method of performing the prediction from left and top pixels are prescribed. In addition, there is a prediction method for each of the brightness and the color difference; such prediction methods for the brightness and the color difference are sometimes different from each other. The other intra-picture prediction techniques include ones of the Patent Documents 1 and 2. According to the documents, a prediction image is produced by specifying a reference position in the same picture by a vector. In other words, a high encoding efficiency can be realized for periodic discrete patterns by using an image currently in the process of coding/decoding for the prediction, instead of an image already coded/decoded in the inter-picture prediction. In the Patent Document 1, the coding efficiency is increased by making the unit for creating a prediction image variable, e.g. making it the unit of 16×16 or 8×8. Further, in the Patent Document 2, a not-yet-coded/decoded area (hereinafter referred to as “not-yet-coded area”) included in a reference range is interpolated by use of a multiplied vector, thereby suppressing the reduction in the encoding efficiency in the case of making reference to a range where many not-yet-coded areas are included. This prediction method is hereinafter referred to as “vector prediction”.